Definition of sly adjective from the Oxford Advanced Learner's Dictionary

     

    sly

     adjective
    adjective
    BrE BrE//slaɪ//
     
    ; NAmE NAmE//slaɪ//
     
    Dishonest
     
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  1. 1(disapproving) acting or done in a secret or dishonest way, often intending to trick people synonym cunning a sly political move (humorous) You sly old devil! How long have you known? She’s a sly one—never lets on what she’s thinking. See related entries: Dishonest
  2. 2[usually before noun] suggesting that you know something secret that other people do not know synonym knowing a sly smile/grin/look/glance, etc.
  3. Word Origin Middle English (also in the sense ‘dexterous’): from Old Norse slœgr ‘cunning’, originally ‘able to strike’ from the verb slá.Idioms secretly; not wanting other people to discover what you are doing He has to visit them on the sly.
See the Oxford Advanced American Dictionary entry: sly